1. Technical Field
The present invention concerns corrosion resistant conductive parts. More specifically, the invention concerns parts, which are resistant to corrosion and electrically conductive, produced by direct plating (hereinafter referred to as simply “plating” or “plate”) of gold (hereinafter, “Au”) on a substrate sheet of a metal such as stainless steel. The “direct” plating means the plating without any other plated layer between the precious metal and the metal substrate. The main use of the corrosion resistant conductive parts according to the invention is metallic separator of fuel cells.
2. Prior Art
For the purpose of maintaining corrosion resistance and decreasing electric contact resistance of metal separators as well as electric collectors of fuel cells there has been proposed to plate Au as a thin layer on a metal substrate such as stainless steel (Japanese Patent Disclosure No. 10-228914). According to the proposal, an Au layer of 0.01 to 0.06 μm thick directly plated on a stainless steel substrate may withstand nitric acid aeration test (JIS H8621). Even after elapse of one hour no  dissolution of chromium is observed, and therefore, no pinhole is considered to be formed.
However, in practical polymer electrolyte fuel cells metal separators are, due to such a high working temperature near 100° C., subjected to severer environment, and therefore, such higher corrosion resistance that the separators withstand boiling sulfuric acid of pH 2, and that even after 168 hours immersion no substantial dissolution of metal ions occurs is required. The problem can be solved by increasing thickness of the plated layers. However, fuel cell stacks require piling up of many metal separators and thus, thickness of the plated layer must be 100 nm or less for practical use in view of the manufacturing costs. A targeted thickness of the plated layer is 20 nm or less.
The inventors pursued the reason why corrosion resistance of the conventional Au-plated products is insufficient, and learned that there exists unexpectedly much amounts of impurities on the surfaces of the metal substrates and in the plated thin Au layers, which causes formation of pinholes in the Au thin layers to lower the corrosion resistance of the thin layers, and that there is an intermediate layer containing the impurities between the thin Au layers and the metal substrates, which weakens the adhesion of the thin layers to the metal substrates.
The inventors made research and development to seek after corrosion resistant conductive parts, particularly, metal separators of fuel cells prepared by disposing a precious metal thin layer on a metal substrate with very few  pinholes in the thin layer, which has dense structure and strongly adhered to the metal substrate, and therefore, can stand with severe environment of using. They discovered that the amounts of the impurities in the Au layers must be regulated to the following limits, C: up to 1.5%, P: up to 1.5%, O: up to 1.5%, S: up to 1.5% and C+P+O+S: up to 4.0%. The discovery matured in an invention was already disclosed (Japanese Patent Application 2002-312226).
In order to obtain such an appropriate plated Au layer technology of cleaning surface is important. It is necessary to remove contaminated film covering the metal substrate by physical and/or chemical means so that a clean surface may be exposed, and to form an Au plated layer immediately thereafter prior to occurrence of the second contamination of the surface. Cleaning by electropolishing is suitable for this purpose. These facts were also disclosed in connection with the above invention.
As the further study the inventors searched the conditions for providing high corrosion resistance to the corrosion resistant conductive parts having a plated Au layer of such a thickness as 20 μm or less, the inventors obtained some new knowledge.